Radio frequency phase shifters are generally of one of two types. The first type uses relative movement between conductive strips to adjust the path length between ports. The second type alters the dielectric properties between first and second conductive elements. By their nature dielectric phase shifters must be much larger than phase shifters of the first type to provide an equivalent phase shift. This results in increased size and cost.
A common type of variable differential phase shifter of the first type is the wiper-type variable differential phase shifter as shown in FIG. 1. The phase shifter consists of a wiper arm 4 that is rotatable about a pivot coupler 5 so that the distal end of the wiper arm scans an arc shaped conductive strip 6. The coupling between the distal end of the wiper arm and the conductive strip is capacitive. The wiper arm is in the form of an impedance transformer to match the input and output impedances of the ports. It will be appreciated that if the wiper arm is rotated about the pivot coupler the path length between port 1 and ports 2 and 3 changes so as to vary the phase of signals output at ports 2 and 3. However, the output ports are directly connected and so there is poor isolation between output ports. In particular, if a signal is injected at one of ports 2 or 3 only half will go to port 1, with the rest either being reflected or transmitted to the other port.
U.S. Pat. No. 6,850,130 discloses phase shifter assemblies enabling simultaneous adjustment for four or more output ports. However, these phase shifters are based upon the standard variable differential phase shifter and provide poor isolation between output ports.
Wilkinson dividers are well-known and are commonly used to provide an isolated power divider using physically fixed components. However, none of the prior art phase shifters incorporate an isolation element.
Variable differential phase shifters are useful in adjusting down tilt, azimuth and beam width of antenna arrays. This is particularly useful to adjust beam tilt, beam width and azimuth for cellular communications antennas. Examples of feed networks and antennas utilizing variable differential phase shifters are shown in WO 02/05383, filed Jul. 10, 2001, entitled Cellular Antenna, and U.S. Ser. No. 10/367,055, now U.S. Pat. No. 6,922,169, issued Jul. 26, 2005, the disclosure of which is hereby incorporated by way of reference.
It would be desirable to provide a variable differential phase shifter having improved isolation between output ports as well as good return loss and antennas utilizing such phase shifters having improved radiation patterns.